Film resistor and method



Dec. 2, 1969 5. o. DORST 3,481,779

FILM RESISTOR AND METHOD Filed July 22, 1966 Deposit A Coal 0121A BefracZm qBm y Dez/b'iz'lfybg The Surface Of The CoaladBOq'fl Clea/lily TheTreated Surface Deposit A Film On The Cleaned Sui/ace II/I/I/I/ Efiem'ir'zfied mzdEZc/zed Surface United States Patent US. Cl. 117-213 9Claims ABSTRACT OF THE DISCLOSURE A resistive film is provided on adevitrified and etched surface of a refractory material having glassyand crystalline phases. The surface is prepared by the deposition of aresistive coating on the surface and then burning off the coating.

The present invention relates to film resistors and more particularly toan improved surface for the bodies carrying resistance films inelectrical resistors and a process for producing the improved surface.

Metal film resistors are made up by the application of a thin film ofmetal to a supporting surface of dielectric material. The metal film isa conductive layer having desired ohmic characteristics. The substancesused for supporting the resistive films include, for example, electricalporcelain or alumina having deposited thereon suitable compositions in afilm; such as, for example, Nichrome to provide desired ohmic values.

The metal film is deposited on the surface by a procedure providinguniform ohmic characteristics across the resistive film, such as byevaporation onto the supporting surface. The metal film may be depositedon refractory material having glassy and crystal phases where materialsare required which retain high structural strength at elevatedtemperatures and have good dielectric properties.

It is desirable to provide superior resistance characteristics in thefilms of the resistors. An increase in ohmic value is desirable withoutthe sacrifice of electrical characteristics in the resultant resistorunit. An increase in ohmic value permits a greater resistance per unitvolume. While the increase of apparent surface area of the supportingdielectric material is desirable and tends toward increased ohmic value,previously the apparent increase of surface area has been susceptible tochange during subsequent processing of the unit resulting in diminutionof the resistive properties in the final structure.

It is an object of this invention to provide a film resistor havingsuperior resistive and electrical properties.

It is another object of this invention to provide an improved refractorybody having glassy and crystal phases for a resistive film.

It is a further object of this invention to produce a metal filmresistor having a higher resistance per unit volume.

Another object of this invention is to provide a process for making athin film electrical resistor with good electrical characteristics on aceramic body having glassy and crystal phases.

Still another object of this invention is to provide a process forpreparing micro-crystalline surfaces for resistive films.

It is a still further object of this invention to provide a simple andeffective process for preparing a large number of film resistors in asingle manufacturing procedure.

In general, this invention involves a resistive film on a devitrifiedand etched surface of a refractory material having glassy and crystalphases providing a film resistor having improved ohmic values. Theresistive film, such as, Nichrome, is applied to the refractory surfaceafter 3,481,779 Patented Dec. 2, 1969 preparation of the surface by thedeposition of carbon and burning the carbon coat off, ultrasoniccleaning the treated surface and finally metallizing the preparedbodies, preferably while rotating.

In a drawing which illustrates this invention, FIGURE 1 is a flow sheetof the process, and FIGURE 2 is a crosssection of the resistor withidentification of the devitrified and etched surface.

The surface of the essentially noncrystalline glassy phase is modifiedto an apparent crystalline state by devitrification while the crystalphase of the refractory material is etched at the grain boundaries atthe surface. In the vitreous state the solid glassy phase is essentiallynon-crystalline and may be thought of as a disordered amorphous solidwith a glassy surface. The process of this invention devitrifies thissurface to a state resembling the crystalline state.

More particularly, a ceramic core having glassy and crystal phases hasthe surface modified by a thermal devitrification and etch process. Thesurface is first modified so that the glassy surface is devitrified andthen the surface is cleaned to remove material. The surface is arefractory dielectric material then ready for the application of themetal film resistance element by a suitable procedure.

A preferred processing of the refractory involves coating the refractorysurface in a closed system to provide a coating of devitrified materialon the surface. After coating, the coated refractory body is cooled andthen immediately heated in the closed reaction chamber in the presenceof oxygen. The treated structures are then cleaned to remove materialloosened in the process. The cleaning leaves the surface ready toreceive the deposition of a resistive film. The surface of the glassyphase is apparently devitrified.

More specifically, this invention provides the superior resistanceelement on a ceramic core by first rotating the cores in large batchesin a closed reaction chamber in the closed system while suitably coatingthe core surfaces with a carbon coating, such as by pyrolysis of ahydrocarbon gas in an inert carrier in the closed chamber. Thetemperature in the reaction chamber is then reduced under the passage ofinert gas but otherwise closed. As soon as the cooling is complete, thecarbon coat is burned off the surfaces in a flow of oxygen-containinggas by raising the closed chamber temperature. Next the treated coresare ultrasonically cleaned to remove loose material after which aresistive film is applied to the treated cores by metallization whilerotating the chamber containing the cores.

Apparatus for the practice of this invention is primarily a reactionchamber in a closed system which may for example, be a quartz bottle andin which the cores to be processed are placed. Conduits open into thechamber for admitting active and inert gases and valves are provided forsealing the chamber.

As an example of the process of surface improvement, reference is madeto the production of Nichrome film resistors on ceramic cores. The coresare placed in a rotating reaction chamber in a closed system and heatedwhile hydrocarbon gas in an inert carrier such as nitrogen is introducedand striking the hot cores and pyrolyzes to deposit a carbon coating onthe core surface. Upon completion of the coating, the hydrocarbon gas isturned olf and cool oxygen-free nitrogen continues to flow attemperatures to gradually cool the cores in the closed chamber. Then,while the chamber remains closed, the carbon coated rotating cores aresubjected to reheating at a temperature in the range of l500 to 1900 F.in a passage of oxygen-containing gas through the system for about fourhours to burn off the carbon. It has been discovered that a superiorimprovement in ohmic value is achieved in holding the carbon coatedsurfaces in the chamber atmosphere standing sealed from the ambientatmosphere and proceeding with the burn-off as soon as the cooling iscomplete.

The cores are ultrasonically cleaned. This completes I the preparationof the surface for the reception of the metal film.

The prepared cores are then preferably strung on rods which are mountedso that the carriage rotates on its axis revolving the cores around acommon axis and are mounted so that they revolve while a Nichromefilament centrally located is heated to a vaporization temperature andis deposited on the moving cores.

EXAMPLE I Ceramic cores for metal film resistors are placed in a closedquartz bottle and heated. A methane gas mixture in nitrogen isintroduced into the quartz bottle through a closed system and passedover the heated cores. Pyrolysis of the methane gas at the pre-heatedcore surfaces deposits a carbon coating on the core surfaces. The quartzbottles are continuously rotating during the passage of the methane gasmixture the pyrolysis and carbon coating. After the deposition of carboncompletely covers the tumbled cores with a carbon coating, the methanegas is turned OE and a cooled oxygen-free nitrogen gas flows into thequartz bottle and cools the cores gradually in an atmosphere sealed fromthe atmosphere. The cooling off under the nitrogen gas flow is continuedfor 45 minutes. The closed quartz bottles are still rotated while oxygenis passed through the system and the cores are reheated to a temperatureof about 1840" F. for about four hours and the carbon coating is burnedoff. The surface in the glassy phase is devitrified and the surface inthe crystal phase is etched. The bottles are then opened and the coresare then ultrasonically cleaned after which the cores are strung on rodsmounted in a rotating carriage which is revolved around an axis. Thecarriage is rotated on its axis to revolve the cores while each corerotates on its own axis. A Nichrome filament centrally located in therotating carriage is vaporized by heating to a vaporization temperatureand then a Nichrome composition is deposited on the moving cores whichare maintained at a lower temperature to receive a deposition of theNichrome film.

The gain in ohmic value of the resistive film which is produced isseveral times the ohmic value of a Nichrome resistive film on a similaruntreated ceramic core.

EXAMPLE II The method of Example I is utilized to produce a carbon coaton the surface of ceramic cores, and the cores sealed in the quartzbottles against the introduction of oxygen-containing gas are cooledunder a flow of nitrogen gas for 45 minutes. The sealed quartz bottlesare still rotated while the carbon coated cores are reheated to atemperature of about 1500 F. for about four hours. The carbon coating isburned off leaving the core surface devitrified in the glassy phase andetched in the crystal phase. The cores are then ultrasonically cleanedand coated with a Nichrome composition utilizing the method of ExampleI. The resistive film thus prepared has a gain in ohmic value of severaltimes that of a resistive film applied to an untreated ceramic core.

EXAMPLE III The method of Example I is utilized to produce a resistiveNichrome film on a batch of 1,000 cores in a single container. A greaterincrease in ohmic value is obtained in the resistive film thus produced.

It will be understood that the details of procedure above set forth mayvary widley within the skill of those engaged in this art. The followingadditional details will assist'in the understanding and application ofthe invention.

Continuous rotation is preferred. Although the tumbling of the coresduring processing may be reduced to a periodic rotation of the containerat intervals of several minutes, the gain in ohmic value issubstantially decreased as a consequence.

Among other advantages, the process of this invention provides a metalfilm resistor having superior resistive properties with adequate orsuperior electrical characteristics; such as, a low noise factor.

This invention applies to metal film resistors, such as, high precision,high stability metal film resistors over a range of resistances. Thisdevelopment, however, is of particular interest in application to thehigher resistance values in the range of standard resistance values, asfor example, resistances in the megohm ranges. The resistors of thisinvention have a satisfactory temperature coefficient of resistance,such as for example, around 75 p.p.m./ C. through a wide temperaturerange. The novel articles embodying useful aspects of this inventioninclude a metal film resistor incorporating a high resistance value on asmall ceramic core. The practical effect of the increased ohmic value isbetter component density. At the same time other important propertiesand characteristics of high precision, high stability metal filmresistors are maintained.

It will be understood that the reference to the production of Nichromefilm resistors on electrical porcelain cores is by way of example of theprocess of surface improvement. Further modifications are within thespirit of this invention. For example, cooling of the carbon coatedcores under the passage of oxygen-free nitrogen for 45 minutes ispreferred, but lesser periods of cooling in the specified procedureprovides substantial increases in ohmic value.

What is claimed is:

1. A method of producing a film resistance element on the surface of arefractory body having both a glassy phase and a crystal phasecomprising the steps of at least:

(a) depositing a resistive coating on the surface of the glasy phase ofthe body;

(b) burning off the coating to devitrify the surface of the glassyphase;

(c) cleaning the article surface;

(d) depositing a resistor film on the clean surface including the glassyphase.

2. The method of improving the ohmic value achieved with a resistivefilm on a surface area of a refractory body having both a glassy phaseand a crystal phase which comprises providing said refractory bodies,preheating the bodies to a temperature in excess of a hydrocarbonpyrolyzing temperature, exposing the preheated bodies to a hydrocarbongas mixture to pyrolze the hydrocarbon and cause a carbon layer todeposit on the surfaces of the bodies, cooling the coated bodies,heating the coated bodies in oxygen-containing gas to burn off thecarbon, cleaning the treated surfaces after carbon burn otf, anddepositing a resistor film on the clean surface.

3. The method of improving a resistive film as set forth in claim 2 inwhich the carbon coated body is heated to a temperature of at least 1500F. and the carbon is burned off.

4 4. The method of improving a resistive film as set forth in claim 2which is accompanied by etching the surface of the crystal phase of saidbodies.

5. The method of improving a resistive film as Set forth in claim 2 inwhich the carbon burn off is con ducted without exposing the bodies toambient air following the deposition of the carbon layer.

6. A metal film resistor including a non-conducting substrate having aglassy phase and a crystal phase, the surface of the glassy hase beingdevitrified and the surface of the crystal phase being-etched and a thinfihn resistance element on said substrate having stable ohmiccharacteristics.

7. The metal film resistor of claim 6 wherein the substrate consists ofa ceramic.

8. The metal fihn resistor of claim 6 wherein the devitrified surface ofthe glassy phase is micro-crystalline.

9. The metal film resistor of claim 6 wherein the resistance elementconsists of Nichrome.

References Cited UNITED STATES PATENTS 3,266,912 8/1966 Murphy .1061

WILLIAM L. JARVIS, Primary Examiner U.S. C1.X.R.

